Bonding of thermal tile insulation

ABSTRACT

An insulative body having first and second porous insulation members and a ceramic binder. Each of the first and second porous insulation members is formed of a fibrous, low-density silica-based material and cooperatively defines a joint. The ceramic binder is disposed between a pair of mating surfaces that form the joint. The ceramic binder couples the first and second porous insulation members together.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. applicationSer. No. 09/676,682, filed Sep. 29, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates generally to thermal insulationtiles and more particularly to a method for bonding thermal insulationtiles.

BACKGROUND OF THE INVENTION

[0003] Thermal insulation tiles, such as those used to insulate thespace shuttle orbiter, are typically formed from low-density fibrousmaterials having extremely high temperature resistance and a relativelylow coefficient of thermal expansion as compared to metals. Thesematerials are well known in the art and include, for example, FRCI(fibrous refractory composite insulation) and AETB (alumina enhancedthermal barrier) materials.

[0004] In fabricating the tiles, fibers of an insulating material, suchas silica, alumina boro-silicate and alumina, are mixed with water toform a slurry. The slurry is deposited into a casting tower where thewater is drained and the silica fibers are subjected to compressiveforces to form a raw block of insulation material having across-sectional area that may range from 144 square inches to almost 576square inches depending upon the dimensions of the casting tower. Theraw block is then dried in an oven and subsequently fired (sintered) tobond the fibers of the insulating material together. Thereafter, tilesare formed from the fired block through conventional machining processeswherein tiles of a desired shape are cut from the solid block.

[0005] One drawback associated with this process is the maximum size ofthe tiles that can be formed. As the surface of the space shuttleorbiter, for example, is relatively large, it is highly desirable toform the tile as large as possible so as to reduce the labor that isrequired to affix the tiles to the orbiter, as well as minimize the useof the material which bonds the tiles to the orbiter to thereby minimizethe weight of the orbiter's thermal protection system. In covering aleading or trailing edge of a craft, a tile having a length in excess of6 feet is highly desirable.

[0006] To some extent, the size of the tiles may be increased byenlarging the size of the casting tower. In practice, however, castingtowers that produce raw blocks having dimensions greater than 22″×22″×7″inches are not practical due to the increased rate at which defects andother problems are encountered in the manufacturing process. Problemssuch as weight associated with transporting a large block filled withwater, the inability to completely dry very large raw blocks,overheating the exterior portion of the raw block during the firingoperation and under heating the interior portion of the raw block duringthe firing operation frequently lead to defects such as shrinking,cracking and improper bonding of the fibers. As the material that isused to form the raw blocks is relatively expensive, the increased rateof defects renders the formation of relatively large fired blockscommercially impracticable.

[0007] Another drawback associated with the previously known methods offorming tiles concerns the manner in which tiles having a complex shapeare formed. Tiles which are relatively flat and sized approximatelyequal to the cross-section of the fired block are relatively easy tomachine with little waste. Tiles having a complex shape, however, areroutinely carved from a fired block, with the remainder of the firedblock being discarded as scrap. As mentioned above, the material that isused to form the raw blocks is relatively expensive. Consequently, tilesthat are produced in a process wherein large amounts of the fired blocksare scrapped are extremely costly to produce.

[0008] Accordingly, there remains a need in the art for a method forforming relatively large insulation tiles. There also remains a need inthe art for a method for forming a complex shaped insulation tile whichproduces relatively less scrap. There also remains a need in the art fora method for bonding insulation tiles together.

SUMMARY OF THE INVENTION

[0009] In one preferred form, the present invention provides aninsulative body having first and second porous insulation members and abinder. Each of the first and second porous insulation members is formedof a fibrous, low-density silica-based material and cooperativelydefines a joint. The binder is disposed between a pair of matingsurfaces that form the joint. The binder couples the first and secondporous insulation members together.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Additional advantages and features of the present invention willbecome apparent from the subsequent description and the appended claims,taken in conjunction with the accompanying drawings, wherein:

[0011]FIG. 1 is a perspective view of an insulative body formed inaccordance with the teachings of the present invention;

[0012]FIG. 2 is a schematic illustration of the method of the presentinvention;

[0013]FIG. 3 is a side elevation view illustrating an alternate jointconstruction;

[0014]FIG. 4 is a perspective view of another insulative body formed inaccordance with the teachings of the present invention;

[0015]FIG. 5 is a side elevation view of the leading edge of a craftformed from insulative materials in accordance with the teachings of thepresent invention; and

[0016]FIG. 6 is a side elevation view of a leading edge of a craftformed in accordance with the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] With reference to FIGS. 1 and 2 of the drawings, an insulatingbody constructed in accordance with the teachings of the presentinvention is generally indicated by reference numeral 10. Insulatingbody 10 is shown to include first and second tile members 14 and 16,respectively, and a ceramic/organic thermal setting binder 20. First andsecond tile members 14 and 16 are extremely porous, being constructedfrom a fibrous, low-density silica-based material. First and second tilemembers 14 and 16 are preferably formed from a homogeneous aluminaenhanced thermal barrier (AETB) material which is well known in the artand more fully described in Leiser et al., “Options for ImprovingRigidized Ceramic Heatshields”, Ceramic Engineering and ScienceProceedings, 6, No. 7-8, pp. 757-768 (1985) and Leiser et al., “Effectof Fiber Size and Composition on Mechanical and Thermal Properties ofLow Density Ceramic Composite Insulation Materials”, NASA CP 2357, pp.231-244 (1984). As those skilled in the art will understand, however,first and second tile members 14 and 16 may be formed from other fibrouslow-density silica-based materials including, for example, fibrousrefractory composite insulation (FRCI), which is well known in the artand more fully described in U.S. Pat. No. 4,148,962, the disclosure ofwhich is hereby incorporated by reference as if fully set forth herein.

[0018] As best shown in FIG. 2, the ceramic/organic thermal settingbinder 20 includes a ceramic binder 22 and a thermal set organic binder24. The ceramic binder 22 may be glass, such as Ferro Corporation'sEG0002, EG1001 and EG2790 electronic glasses, or ceramic, such as analuminum oxide and silica mixture, and is selected on the basis of itscoefficient of thermal expansion. Preferably, the coefficient of thermalexpansion of the ceramic binder 22 is about equal to the coefficient ofthermal expansion of the first and second tile members 14 and 16.

[0019] The thermal set organic binder 24 is an agent that aids in theprocessing of insulating body 10. In this regard, the thermal setorganic binder 24 is an agent that is employed to temporarily bond thefirst and second tile members 14 and 16 to one another. Additionally,the thermal set organic binder 24 is an agent that assists in thedistribution of the ceramic binder 22 as will be discussed in greaterdetail, below. Examples of suitable thermal set organic binders 24include epoxies and acrylics.

[0020] The thermal set organic binder 24 and ceramic binder 22 arecombined and preferably thinned out (i.e., the thermal set organicbinder 24 is at least partially dissolved) to a desired viscosity with asuitable solvent 26. The ceramic/organic thermal setting binder 20 isthen applied to the pair of mating surfaces 30 that form the joint 32between the first and second tile members 14 and 16. In applying theceramic/organic thermal setting binder 20 to the joint 32, it is highlydesirable that the mating surfaces 30 are sufficiently wetted out so asto create a high quality bond. It is also desirable that theceramic/organic thermal setting binder 20 not significantly wick intothe porous and fibrous tile members 14 and 16 as relatively thinnerbonds are more desirable (thinner bonds are lighter and less likely toaffect the thermal and mechanical properties of the finished insulatingbody 10 as compared to relatively thicker bonds).

[0021] Once a desired amount of the ceramic/organic thermal settingbinder 20 has been applied to the mating surfaces 30, the matingsurfaces 30 are placed in contact with one another, the thermal setorganic binder 24 bonds the first and second tile members 14 and 16together and a tile assembly is formed. Measures may be taken to ensurethat the mating surfaces 30 will remain in constant and continuouscontact with one another during the subsequent processing step. Suchmeasures are well known in the art and are typically employed in thefabrication of epoxy composites. One such measure is the use of a vacuumbag wherein the first and second tile members 14 and 16 are placed in avacuum bag, the vacuum bag is coupled to a vacuum source (e.g., a vacuumpump) and air is evacuated from the vacuum bag to permit the atmosphereto exert pressure onto the joint 32 to maintain the mating surfaces 30in constant and continuous contact with one another. Other measuresinclude the use of an adhesive tape or the application of a small weightacross the length and width of the joint, so as to exert a force ontothe joint which maintains the mating surfaces 30 in constant andcontinuous contact with one another.

[0022] The tile assembly is next placed into an oven and slowly heated.As the temperature of the thermal set organic binder 24 increases, itsviscosity lowers, permitting it to flow into through the joint 32 andinto any open pores in the first and second tile members 14 and 16. Asthe thermal set organic binder 24 is mixed with the ceramic binder 22,the flowing action of the thermal set organic binder 24 assists in thedistribution of the ceramic binder 22 by carrying the ceramic binder 22through the joint 32 and into the open pores. Depending upon theparticular type of thermal set organic binder 24 that is used, thethermal set organic binder 24 will set up at about 200-500° F., therebyfixing the position of the particles of the ceramic binder 22 that aredistributed throughout the joint 32 and temporarily bonding the firstand second tile members 14 and 16 to one another. The tile assembly maythen be removed from the oven to permit any vacuum bags, weights, tape,etc. to be removed. Thereafter, the tile assembly is placed in a furnacewhere it is slowly heated to a temperature of about 800° F. to about1000° F. and held within this temperature range for an appropriate timesuch as one hour, for example, to permit the thermal set organic binder24 to burn-out of the joint 32 so as not to affect the weight, strength,thermal properties or coefficient of thermal expansion of the joint 32.Thereafter, the tile assembly is slowly heated to a temperature fromabout 1200° F. to about 2400° F. to permit the ceramic binder 22 to fusethe into the first and second tile members 14 and 16 and fixedly couplethe mating surfaces 30 to one another.

[0023] Preferably, a surface hardening agent 40 is applied to the matingsurfaces 30 of the joint 32 and cured prior to the application of theceramic/organic thermal setting binder 20. Examples of suitable surfacehardening agents 40 include silica sol and alumina sol. The curedsurface hardening agent 40 is operable for partially filling the voidsin the mating surfaces 30 to thereby limit the ability of theceramic/organic thermal setting binder 20 to wick into the first andsecond tile members 14 and 16. If silica sol is employed as the surfacehardening agent 40, it preferably includes small silica particles in thesize range of from about 4 nanometers to about 150 nanometers. Thesilica particles are mixed with a carrier liquid, such as water with asmall amount of ammonia such that the silica particles are present in anamount of from about 15 parts by weight to about 50 parts by weight ofthe mixture of silica and liquid, producing a mixture having theconsistency of water. One operable silica sol of this type iscommercially available as Nalco 2327 manufactured by Nalco ChemicalCompany. Curing is accomplished by heating the first and second tilemembers 14 and 16 to an elevated temperature, such as 300° F. until theliquid carrier has completely evaporated. If desired, a pore-obstructingmaterial 41, such as cordierite or mullite, may be applied to the matingsurfaces 30 of the joint 32 prior to the application of the surfacehardening agent 40 to limit the depth with which the surface hardeningagent 40 is permitted to penetrate.

[0024] Also preferably, the first and second tile members 14 and 16 areformed with mitered end portions 50 so that the mating surfaces 30 areangled with respect to the exterior surfaces 60 of the first and secondtile members 14 and 16. Construction of insulating body 10 in thismanner increases the surface area of joint 32 as compared to aconventional butt joint 32 as illustrated in FIG. 3, to thereby increasethe strength of insulating body 10 in the area of the joint 32. Alsoadvantageously, the angling of the mating surfaces 30 relative to theexterior surfaces 60 permits any adverse effects of the ceramic/organicthermal setting binder 20 to be distributed over a path that is skewedto the direction through which thermal energy is transmitted throughinsulating body 10.

[0025] The following non-limiting examples describe the inventionfurther and represent best modes for practicing the invention.

EXAMPLE I

[0026] Tile members 14 and 16 are formed from AETB with mitered endportions 50. A silica sol having silica particles that are mixed with acarrier liquid, such as Nalco 2327 which is commercially available fromNalco Chemical Company and 27 percent by weight of cordierite powderfrom Ferro Corporation, is applied to the mating surfaces 30 that areformed into the mitered end portions 50 of the tile members 14 and 16.The tile members 14 and 16 are dried at about 300° F. for about 30minutes to harden the mating surfaces 30 and to substantially reduce theporosity of the mating surfaces. An ceramic/organic thermal settingbinder 20 consisting of about 90 percent by weight of a ceramic binder22 and about 10 percent by weight of a thermal set organic binder 24,such as an acrylic is provided. In the particular example provided, theceramic binder 22 has a composition of about 0 percent by weight toabout 90 percent by weight of aluminum oxide and 100 percent by weightto about 10 percent by weight of silica and preferably about 83.3percent by weight of aluminum oxide and about 16.6 percent by weight ofsilica. The ceramic/organic thermal setting binder 20 is mixed with anacetone solvent to form a liquid having the consistency of milk. One ormore coats of the ceramic/organic thermal setting binder 20 are appliedto each of the mating surfaces 30 and the mating surfaces 30 are placedin contact with one another. The tile assembly is placed in a vacuum bagand a source of vacuum is applied to the vacuum bag to remove the airtherefrom. The tile assembly is heated slowly in an oven to first 180°F. then to about 350° F. to cause the acrylic to temporarily bond thetile members 14 and 16 to one another. The tile assembly is removed fromthe oven, the vacuum bag is removed from the tile assembly and the tileassembly is thereafter heated slowly in a furnace such that thetransition between approximately 800° F. to approximately 1000° F. ismade in about one hour to permit the acrylic to burn out of the joint32. The tile assembly is thereafter fired in the furnace at atemperature of about 1200° F. to about 2400° F., and preferably at about2000° F., to fuse the ceramic binder (22) to the mating surfaces andfixedly couple the tile members 14 and 16 together.

EXAMPLE II

[0027] Tile members 14 and 16 are formed from AETB with mitered endportions 50. An ceramic/organic thermal setting binder 20 consisting ofabout 95 percent by weight of a ceramic binder 22, such as FerroCorporation electronic glass EG002, EG1001 or EG2790, and about 5percent by weight of a thermal set organic binder 24, such asethocellulose, is mixed with a butyl carbitol acetate solvent to form apaste that is somewhat wetter than commercially available tomato paste.A first coat of the ceramic/organic thermal setting binder 20 is appliedto each of the mating surfaces 30 and permitted to slightly wick intothe tile members 14 and 16. A second coat of the ceramic/organic thermalsetting binder 20 is thereafter applied to the mating surfaces 30 andthe mating surfaces 30 are placed in contact with one another. A smallweight is applied to the assembly to ensure that the mating surfaces 30remain in constant contact during the subsequent step. The tile assemblyis then heated slowly to about 350° F. to set the ethocellulose andtemporarily bond the tile members 14 and 16 to one another. The weightis thereafter removed and the tile assembly is heated slowly in afurnace such that the transition between approximately 800° F. toapproximately 1000° F. is made in about one hour to permit theethocellulose to burn out of the joint 32. The tile assembly isthereafter fired in the furnace at a temperature of about 2000° F. toabout 2400° F. to fuse the electronic glass to the mating surfaces andfixedly couple the tile members 14 and 16 together.

EXAMPLE III

[0028] Tile members 14 and 16 are formed from FRCI with mitered endportions 50. A silica sol having silica particles that are mixed with acarrier liquid, such as Nalco 2327 which is commercially available fromNalco Chemical Company, is applied to the mating surfaces 30 that areformed into the mitered end portions 50 of the tile members 14 and 16.The tile members 14 and 16 are dried at about 300° F. for about 30minutes to harden the mating surfaces 30 and to substantially reduce theporosity of the mating surfaces. An ceramic/organic thermal settingbinder 20 consisting of about 90 percent by weight of a ceramic binder22, such as Ferro Corporation electronic glass EG0002, EG1001 or EG2790,and about 10 percent by weight of a thermal set organic binder 24, suchas epoxy, is mixed with an acetone solvent to form a liquid having theconsistency of milk. One or more coats of the ceramic/organic thermalsetting binder 20 are applied to each of the mating surfaces 30 and themating surfaces 30 are placed in contact with one another. The tileassembly is placed in a vacuum bag and a source of vacuum is applied tothe vacuum bag to remove the air therefrom. The tile assembly is heatedslowly in an oven to about 350° F. to cause the epoxy to temporarilybond the tile members 14 and 16 to one another. The tile assembly isremoved from the oven, the vacuum bag is removed from the tile assemblyand the tile assembly is thereafter heated slowly in a furnace such thatthe transition between approximately 800° F. to approximately 1000° F.is made in about one hour to permit the epoxy to burn out of the joint32. The tile assembly is thereafter fired in the furnace at atemperature of about 2000° F. to about 2400° F. to fuse the electronicglass to the mating surfaces and fixedly couple the tile members 14 and16 together.

EXAMPLE IV

[0029] Tile members 14 and 16 are formed from AETB with mitered endportions 50. A ceramic organic setting binder 20, such as NipponElectronic Corporation electronic glass GA-13 is mixed with a butylcarbitol acetate solvent to form a paste that is somewhat wetter thancommercially available tomato paste. A coat of the ceramic organicsetting binder 20 is applied to each of the mating surfaces 30 andpermitted to slightly wick into the tile members 14 and 16. A smallweight is applied to the assembly to ensure that the mating surfaces 30remain in constant contact during the subsequent step. The tile assemblyis then heated slowly to about 200° F. to about 400° F. to evaporate thesolvent and temporarily bond the tile members 14 and 16 to one another.The weight is thereafter removed and the tile assembly by heated slowlyin a furnace such that the transition between approximately 800° F. toapproximately 1000° F. is made in about one hour to permit the thermalset organic binder 24 to burn out of the joint 32. The tile assembly isthereafter fired in the furnace at a temperature of about 1300° F. toabout 1800° F. to fuse the electronic glass to the mating surfaces 30and fixedly couple the tile members 14 and 16 together.

[0030] While the insulating body 10 has been illustrated thus far asbeing a planar insulative tile formed from several substantially planartile members, those skilled in the art will appreciate that theinvention, in its broader aspects, may be constructed somewhatdifferently. For example, insulating body 10′ may be formed from aplurality of fired blocks 70 of porous, fibrous, low-densitysilica-based material as illustrated in FIG. 4. In this arrangement, rawblocks formed in a casting tower are dried and fired in a furnace toproduce fired blocks 70 in a process that is well known in the art. Thefired blocks 70 are thereafter bonded together with a ceramic/organicthermal setting binder 20 of the type and in the manner disclosed aboveto produce a block assembly. The block assembly may thereafter bemachined as desired. The bonding of fired blocks 70 is advantageous inthat it substantially reduces the processing time associated with thepreparation of the mating surfaces 30, the application of theceramic/organic thermal setting binder 20, the curing of the thermal setorganic binder 24 and the subsequent firing to set the ceramic binder22.

[0031] Another arrangement is illustrated in FIG. 5 wherein a pair oftile members 14″ and 16″ are coupled to one another to form the leadingedge 80 of a craft. Construction of the leading edge 80 from a pluralityof planar tile members as illustrated in FIG. 5, as opposed to the priorart method of carving the leading edge 80 a from a monolithic block 84as illustrated in FIG. 6, is both extremely efficient and less costlydue to a substantial reduction in the amount of waste that is generatedto machine the leading edge 80.

[0032] While the invention has been described in the specification andillustrated in the drawings with reference to a preferred embodiment, itwill be understood by those skilled in the art that various changes maybe made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention as defined in the claims. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment illustrated by the drawingsand described in the specification as the best mode presentlycontemplated for carrying out this invention, but that the inventionwill include any embodiments falling within the foregoing descriptionand the appended claims.

What is claimed is:
 1. An insulative body comprising: first and secondporous insulation members, each of the first and second porousinsulation members being formed of a fibrous, low-density silica-basedmaterial, the first and second porous insulation members cooperativelydefining a joint; and a ceramic binder selected from a group of bindersconsisting of glass and ceramic, the binder being disposed between apair of mating surfaces that form the joint, the binder coupling thefirst and second porous insulation members together.
 2. The insulativebody of claim 1, wherein the ceramic binder has a coefficient of thermalexpansion that is about equal to a coefficient of thermal expansion ofthe first and second porous insulation members.
 3. The insulative bodyof claim 1, wherein each of the first and second porous insulationmembers includes a mitered end portion.
 4. The insulative body of claim1, wherein the ceramic binder includes aluminum oxide and silica.
 5. Aninsulative body comprising: first and second porous insulation members,each of the first and second porous insulation members being formed of afibrous, low-density silica-based material, the first and second porousinsulation members cooperatively defining a joint; and anceramic/organic thermal setting binder having a thermal set organicbinder and a ceramic binder selected from a group of binders consistingof glass and ceramic, the ceramic/organic thermal setting binder beingdisposed between a pair of mating surfaces that form the joint; whereinthe thermal set organic binder initially adhesively couples the firstand second porous insulation members together and thereafter isburned-out of the joint when the insulating body is fired to permit theceramic binder to fuse into and fixedly couple the first and secondporous insulation members together.
 6. The insulative body of claim 5,wherein the thermal set organic binder is selected from a group ofthermal set organic binders consisting of epoxies and acrylics.
 7. Theinsulative body of claim 6, wherein the thermal set organic binderincludes ethocellulose.
 8. The insulative body of claim 5, wherein theorganic thermal setting binder further includes a solvent for at leastpartially dissolving the organic binder.
 9. The insulative body of claim5, wherein the ceramic binder has a coefficient of thermal expansionthat is about equal to a coefficient of thermal expansion of the firstand second porous insulation members.
 10. The insulative body of claim5, wherein each of the first and second porous insulation membersincludes a mitered end portion.
 11. The insulative body of claim 5,wherein the ceramic binder includes aluminum oxide and silica.
 12. Aninsulative tile comprising: a first tile member formed of a fibrous,low-density silica-based material, the first tile member defining afirst mating surface; a second tile member formed of the fibrous,low-density silica-based material, the second tile member defining asecond mating surface; and a ceramic/organic binder applied onto atleast one of the first and second mating surfaces such that theceramic/organic binder is juxtaposed between the first and second matingsurfaces, the ceramic/organic binder including an organic thermalsetting binder and a ceramic binder; wherein at least a portion of theinsulative tile has been subjected to an elevated temperature that curesthe organic thermal setting binder to thereby fix the first and secondmating surfaces to one another.
 13. The insulative tile of claim 12,wherein the fibrous, low-density silica-based material is selected froma group consisting of fibrous refractory composite insulation, aluminaenhanced thermal barrier and combinations thereof.
 14. The insulativetile of claim 12, wherein the ceramic binder has a coefficient ofthermal expansion that is about equal to a coefficient of thermalexpansion of the first and second tile members.
 15. The insulative tileof claim 12, wherein the thermal set organic binder is selected from agroup of thermal set organic binders consisting of epoxies and acrylics.16. The insulative tile of claim 12, wherein the pore-obstructingmaterial is selected from a group of pore-obstructing materialsconsisting of cordierite and mullite.
 17. The method of claim 12,wherein the ceramic binder includes aluminum oxide and silica.